The present invention relates to a ball screw. More particularly, the invention relates to a ball screw which is little subject to deterioration of operating characteristics and occurrence of wear, torque change, noise, vibration, etc. due to stagnation or competition of balls.
A ball screw comprises a screw shaft and a nut. An outer surface of the screw shaft and an inner surface of the nut are provided with the respective helical thread grooves corresponding to and confronting with each other. The helical space formed between the two thread grooves, i.e., ball rolling path is filled with a number of balls while allowing the balls to rotate. In this arrangement, the screw shaft and the nut make a relative helical movement with the rolling, of these balls to make a relative movement in the axial direction.
Such a ball screw has a gap formed between the adjacent balls. As the balls roll, the total of all the gaps in one circuit varies with every movement of one ball.
In particular, a ball screw for use in injection molding machine or the like, i.e., ball screw comprising balls having a diameter relatively greater than the diameter of the shaft and hence having a relatively great scooping angle tends to have a great variation of the total of gaps (value obtained by totaling the gaps between the balls over the entire ball screw).
Thus, a ball screw which varies greatly in the total of gaps between balls is subject to deterioration of operating characteristics and occurrence of wear, torque change, noise (noise of ball slipping), vibration, etc. during the rolling of balls (during the operation of ball screw) due to stagnation or competition of balls to disadvantage.